By default only error logs are recorded to reduce verbosity. For debugging purposes, it may be necessary during the development phase to add in your file config/mes_options.php (see the article dedicated to it).
// define the maximum level of verbosity of the logs
define('_LOG_FILTRE_GRAVITE', 8);
The different values of this constant are (from least verbose to most verbose):
- 0 (_LOG_HS)
 - 1 (_LOG_ALERTE_ROUGE)
 - 2 (_LOG_CRITIQUE)
 - 3 (_LOG_ERREUR)
 - 4 (_LOG_AVERTISSEMENT)
 - 5 (_LOG_INFO_IMPORTANTE) (default)
 - 6 (_LOG_INFO)
 - 7 (_LOG_DEBUG)
 
See also
Programmer.spip.net
Note: historical syntax spip_log('message'); without level precision is still supported but deprecated.